The specification of genes controlling the development of muscle cells and specifically the resultant assembly of myofibrillar structures characteristic of the terminal differentiated state are the principal objectives of this program. A large number of mutants specifically affecting the structure and function of body-wall muscle cells in the nematode, Caenorhabditis elegans, have been isolated and genetically characterized. Morphological studies of these mutant muscle structures using light and electron microscopic methods will be critical in dissecting the effects of genetic alteration upon cellular, subcellular and supramolecular organization. The genetic analysis already performed will permit relations between chromosomal position and developmental effects. Different developmental stages will be examined in order to determine the action of genes affecting muscle with respect to developmental time. Several temperature-sensitive mutants that have been isolated will be particularly relevant.